Image sensors used in digital cameras have a spectral response with a non-negligible component in the infrared (IR). This results in opportunities as well as challenges.
An opportunity lies in that during night time, or in low-light conditions the IR-component may provide useful information about the imaged scene. Typically, low light conditions occur during dusk or dawn. Other low light condition scenes are badly lit rooms. Low-light condition may also be defined as conditions where the amount of visible light and IR-radiation are comparable, hence, low-light condition may also be referred to as a mixed light condition.
During night time, or in low-light conditions use may be made of incoming radiation from the IR portion of the spectrum. The IR radiation will not contain any color information, and instead of performing a color separation, the only parameter is the intensity of the incoming radiation, which may be presented as a black and white intensity image (or with any desirable color scale).
A challenge is found during day-time imaging, where the addition of an IR-component may distort the color balance in images captured using a digital camera. In some situations, some or all pixels of the image sensor may even be completely saturated by the infrared component.
A way of maintaining the beneficial effects while suppressing the less beneficial effects is to add a movable IR-cut filter in the beam path in front of the image sensor. In this way, the IR-cut filter may be used during daylight conditions, enabling acquisition of color images. Pixels of the image sensor will then operate in a first manner, where the incident light is divided into colors and detected as a charge on individual photodetectors thus enabling color separation. The IR-cut filter will be removed during night time, or in low-light conditions allowing IR-radiation to reach the image sensor. Pixels of the image sensor will then operate in a second manner, where the only parameter measured by the pixels is the intensity of the incoming radiation. Hence, intensity based images may be captured, and presented in black and white.
There may, however, be instances where it would be desirable to be able to capture color separated images even in low-light conditions.